Acellularized nerve allografts (ANAs) are rapidly gaining clinical popularity. ANAs, cadaveric nerves processed to remove antigenic cellular material, provide the microstructure of a nerve autograft without the morbidity associated with autologous nerve harvest or the host immunosuppression required with fresh cadaveric nerve allografts. These off the shelf nerve substitutes are currently being used clinically for long ga, large diameter (large volume) nerve reconstruction. However, they have only been shown to support nerve regeneration similar to autografts in small diameter nerves across short gaps (small volume). ANAs do not contain Schwann cells (SCs) and therefore depend on proliferation of host SCs into the graft to support axonal regeneration. Our preliminary results demonstrate that SC proliferation is limited in large volume ANAs and is associated with limited axonal regeneration across larger volume ANAs. The elevated proliferative requirement of large volume ANAs either exhausts the SCs ability to replicate or chronically stresses them inducing a senescent state. Senescent cells change the extent and types of proteins they secrete, adopting a senescent-associated secretion phenotype (SASP). The SASP drastically alters the tissue microenvironment surrounding senescent cells. We hypothesize that the SASP of SCs in ANAs alters the environment and negatively affects axonal regeneration. We have shown that as the volume of ANAs increases, the presence of senescent cells within the graft also increases and axonal regeneration across the graft decreases. Thus we postulate that decreased regeneration is due to the increased presence of senescent SCs altering the regenerative microenvironment. Endoneurial tube basal lamina, SCs, and their secreted growth factors, provide a pro- regenerative microenvironment for regenerating axons. The absence of SCs in ANAs limits their maximum regenerative length and the presence of senescent SCs may negatively affect the regenerative microenvironment. The current proposal investigates the regenerative limits of ANAs (Aim 1), and determines the mechanistic role of senescent SCs in limiting regeneration through large volume ANAs (Aim 2).